Problem: Which of these numbers is prime? ${11,\ 63,\ 69,\ 88,\ 93}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 11 are 1 and 11. The factors of 63 are 1, 3, 7, 9, 21, and 63. The factors of 69 are 1, 3, 23, and 69. The factors of 88 are 1, 2, 4, 8, 11, 22, 44, and 88. The factors of 93 are 1, 3, 31, and 93. Thus, 11 is a prime number.